1. Field of the Invention
This invention relates generally to a generator system and, more particularly, to a generator system for a vehicle that includes an active rectifier bridge employing low on-resistance metal oxide semiconductor field effect transistor (MOSFET) switches and a control scheme for controlling the switches.
2. Discussion of the Related Art
Vehicles employ generators that are driven by a belt coupled to the vehicle engine to generate electrical power. Automotive electrical generators typically employ an AC synchronous machine with three-phase stator windings, a DC excitation winding on a claw-pole rotor, a voltage regulator and a three-phase diode bridge rectifier to produce a DC output. The excitation current of the synchronous machine is controlled by the voltage regulator to regulate the DC output voltage of the generator. Permanent magnets have been employed in the claw-pole device to increase the power output and efficiency of the generator for a given generator.
High efficiency generators usually employ shaped conductor stator windings to reduce copper losses, thinner stator laminations to reduce iron losses, low friction bearings, a laminated rotor construction and permanent magnetic excitation, all of which add significant cost and/or mass to the generator.
As mentioned above, known generator assemblies typically employ a three-phase diode bridge to convert the AC current to a DC current, where the diodes conduct depending on the propagation direction of the current through the three-phase coils. A three-phase machine generally requires six diodes. The three-phase diode bridge generally contributes 20% -50% of the total losses of the generator. Particularly, the amount of power used to conduct the diodes may provide a power loss at 100 amps of about 180 watts and a total diode voltage loss of about 1.8 volts.